Parr Instrument 6400 - Manual (Page 59)

Calculations
6400 7
w w w . p a r r i n s t . c o m 57
Spiking Samples
It is sometimes necessary to add a spiking material
to samples which are very small, have a low heat of
combustion, or have a high moisture content to add
sufficient heat to drive the combustion to comple-
tion. Benzoic acid is an excellent material for spiking
for all of the same reasons it is a good standard
material. White oil is also an excellent material,
particularly for liquid samples. The 6400 Calorimeter
can automatically compensate for the addition of
spiking materials to these samples. The calculations
are modified in these cases as follows:
Hc =
WT-e
1
- e
2
- e
3
- (Hcs)(M
s
)
m
Where:
Hcs = The spiking material (cal/g)
M
s
= Mass of spiking material
This factor is added to the calculations when Spike
Controls, Use Spiking is set to ON. Heat of Combus-
tion of Spike is entered as calories per gram. The
controller will prompt the user to enter the weight
of spiking material. Fixed spikes can be used when,
Use Fixed Spike is set to ON and entering the mass
of the spike on - Weight of Fixed Spike.
Conversion to Other Moisture Bases
The calculations described above give the calorific
value of the sample with moisture as it existed when
the sample was weighed. For example, if an air-
dried coal sample was tested, the results will be in
terms of heat units per weight of air-dry sample. This
can be converted to a moisture free or other basis
by determining the moisture content of the air-dry
sample and using conversion formulae published in
ASTM Method D3180 and in other references on fuel
technology.
Conversion to Net Heat of Combustion
The calorific value obtained in a bomb calorimeter
test represents the gross heat of combustion for
the sample. This is the heat produced when the
sample burns, plus the heat given up when the
newly formed water vapor condenses and cools to
the temperature of the bomb. In nearly all industrial
operations, this water vapor escapes as steam in the
flue gases and the latent heat of vaporization, which
it contains, is not available for useful work. The net
heat of combustion obtained by subtracting the
latent heat from the gross calorific value is therefore
an important figure in power plant calculations.
If the percentage of hydrogen H, in the sample is
known, the net heat of combustion, H
net
BTU per
pound can be calculated as follows:
H
net
=
1.8Hc - 91.23H
(Liquid fuels, ASTM D240)
To calculate H
net
for solid fuels please refer to ASTM
D5865.